The present invention relates to a photoelectric length measuring instrument for measuring changes in position of an object, of the type wherein an optical system is provided for projecting the image of an aperture onto the object, and for projecting a reflected image of this aperture back from the object to the instrument.
German Pat. No. 20 23 265 discloses a prior art photoelectric measuring system for the determination of changes in position of an object, in which a periodically oscillating illuminated aperture is provided for scanning the object to be measured. An optical system is provided for imaging the oscillating image of the aperture projected onto the object back on to the oscillating aperture. This oscillating aperture simultaneously functions as a source of illumination light and as a scanning element. The oscillating aperture periodically scans the position of the aperture image in its oscillation plane. The position of this image represents a measure of the change of the position of the object to be measured. In order to allow relatively large position deviations of the object to be measured, the oscillating system is arranged slidably in the field of view of the reproducing optical system. For the determination of the displacement path, this oscillating system is mechanically coupled with a length measuring system.
In this prior art measuring system, the oscillating aperture is fastened to a vibrating or oscillating string. This oscillating system requires for its excitation an electro-mechanical oscillator. The amplitude of oscillation is regulated by means of a velocity sensor which supplies an input signal to a servo system which symmetrically adjusts the oscillating aperture with respect to the perpendicular which extends from a mirror connected to the object to be measured and passes through the center of the reproduction objective lens. Furthermore, a separate length measuring system is required for the determination of the displacement of the oscillating aperture.